1. Field of the Invention
The present invention relates to the field of magnetoelectronics, and more specifically, the present invention relates to a method for patterning magnetic tunnel junctions using low atomic weight ion sputtering.
2. Description of Related Art
Magnetoresistive random access memory (MRAM) devices can be formed using standard patterning techniques. MRAM devices function as integrated circuits. In a Magnetic tunnel junction (MTJ) electrons pass from one magnetic material to another by tunneling through a thin insulating layer. MTJs within an MRAM generally include metals which are difficult to chemically etch. Positive-tone lithography is often used to pattern MTJ devices and halogen etch gases are used to remove the top mask material.
Ion etching is a process using an inert gas by sputtering to remove atoms. In the prior art, etch chemistries include sputtering using different gas chemistries. Generally, inert gases are used to try to improve selectivity. Inert gases typically used include He, Ar, Xe, and N2.
Sputtering using inert gases in either plasma etch chambers, called reactive ion etching (RIE), or ion beam chambers, called ion beam etching (IBE), can be used to pattern magnetic tunnel junctions. However, the low selectivity of physical sputtering processes using inert gases makes it difficult to control the sidewall profile of the patterned junctions. The difficulty is partially due to erosion of the hard mask for patterning.
Ion etching can cause atoms which are removed to be redeposited, creating residue upon the sidewalls of the MTJ. This is undesirable because the residue can negatively affect the device by causing shorts across the tunneling barrier layer of a MTJ, increase resistance, and degrade the magnetic properties of the device.
Methods to avoid and decrease residue and corrosion of the magnetic tunnel junction have been employed in the prior art. Decreasing residue improves the process yield. Such methods generally include modifying sidewall spacer features, using different power levels for etching different layers, and replacing the hard mask material with another material less prone to oxidation or other adverse reactions.
Sputter chemistries with the ability to passivate the sidewall such as CH3OH have been used for MTJ patterning. No corrosion is observed when using CH3OH, but the process requires very high ion energies and the etch rate is very low which is undesirable. Alternative chemistries such as CO/NH3 have been employed. Adding NH3 to pure CO etch plasma increases the etch rates, however, the etch rate is still very low. Additionally, experiments in the literature regarding CO/NH3 chemistry have detailed a need for high temperature conditions to drive volatile product formation and this can potentially degrade the magnetic performance.
The etch rate of a sputtering process is directly related to the sputter yield, which is defined as the number of target material atoms removed per impinging etchant ionic species. Sputter yield is hence a unitless ratio that can be converted to an etch rate through knowledge of the atomic density and surface area of the target surface. It has been reported in the art that the sputter yield of a process is dependent on the ratio between the mass of the target atom and the projectile. See, for example, Guo et al., “Mixing-layer kinetics model for plasma etching and the cellular realization in three-dimensional profile simulator,” J. Vac. Sci. Technol. A 27(2), March/April 2009, the contents of which are incorporated by reference as if fully set forth herein. The sputtering yield is expressed by:Yt_by_p=A(√{square root over (E)}−√{square root over (Eth)})f(θ)  (1)
Yt_by_p represents the sputtering yield coefficient of a target (t) by projectile ion (p). E is the ion bombardment energy. Eth is the threshold energy. A is the linear proportional coefficient, and f(θ) is a function of off-normal angle θ to the angular dependence. A is expressed by:
                    A        =                              0.0054            ⁢                                          (                                                      Z                    p                                    ⁢                                      Z                    t                                                  )                                            1                ⁢                                  /                                ⁢                2                                      ⁢                                          (                                                      M                    t                                                                              M                      p                                        +                                          M                      t                                                                      )                                              -                      0.0198            .                                              (        2        )            
Mt,p is the atomic weight of target and projectile atoms, respectively. Zt,p is the atomic number of target and projectile atoms, respectively. The formula for the threshold energy was recently developed and Eth is expressed by:Eth=25.2(Mt/Mp)−0.6+0.928(Mt/Mp),  (3)
Therefore, there is a need in the art for improved methods for MTJ patterning to increase the sputter yield, leading to better profile control.